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Old 04-07-2020, 09:58 PM   #1
dcs76
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Default AOA Indicator units?

I am doing a 45° DB. Altitude is 6000. speed is 350 KIAS. The FPM is about 1° below the 45° pitch ladder, the ADI shows 45°. TacView shows 45° Pitch and an AOA of about 1°. I am pretty much wings level and have the expected g-load.

The AOA Indicator in the A-10C, however, shows a value of 11,2. I am honestly confused about the units the AOA indicator is using. This can't possibly be the AOA in degrees. It also cannot be in mils as this would be about 0.6° AOA. It is supposed to be in ° as far as I understand. The limit markers on the AOA indicator do also hint at the units being degree. So why would the AOA indexer show me a value of 11 for this? Am I doing something wrong?
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Old 04-07-2020, 10:13 PM   #2
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AoA units are a scaled, arbitrary units proportional to degrees. Your question suggests you may not understand what AoA is, so Ill explain but excuse me if Im mistaken.

AoA is the angle between the horizontal centerline of your aircraft, and the air passing by. It is easiest to understand if you look into ho bits measured: a little vane near the nose, as the vane gets deflected by the passing air, the indexer moves.

So in some way it has nothing to do with dive angle. In theory if you were going fast enough you could be pitched 45 degrees nose down and have a zero AoA.

Practically you care about your AoA during maneuvers and landing, and its sufficient to know the three bands during landing, and your peak performance tone is enough during maneuvering. I cant actually think of a time Ive used the AoA indexer gauge during flight.
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Old 04-07-2020, 10:53 PM   #3
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The AoA indicator does indeed use arbitrary "units" that are not degrees, just like Puma says.

Looking at these discussions, it seems "units of AoA" get a lot of people confused, and there's also no commonality between different aircraft or different manufacturers:

https://www.reddit.com/r/hoggit/comm...in_units_isnt/

https://www.pprune.org/military-avia...le-attack.html

https://forums.eagle.ru/showthread.php?t=64172

https://airwarriors.com/community/th...-of-aoa.15502/
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Old 04-07-2020, 11:06 PM   #4
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Quote:
Originally Posted by Puma View Post
AoA is the angle between the horizontal centerline of your aircraft, and the air passing by.
Just to add a little more precision to your explanation: Angle of Attack is the angle between the chord line of the wing airfoil and the local relative wind.
The chord line is an imaginary straight line drawn between the leading and trailing edges of an airfoil.


It's a common misconception to measure the angle in relation to the aircraft's fuselage structure. But the reality is that the fuselage has no bearing on the measurement of AOA, just like flight path has no bearing on AOA. It is entirely referenced to the wing orientation, and the direction of the airflow that meets that wing.
It also applies to propeller blades, rotor blades, fan blades, the empennage (tail) and any aerodynamic surface moving through a medium, or across which a medium flows.


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Old 04-08-2020, 03:01 AM   #5
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Quote:
Originally Posted by randomTOTEN View Post
Just to add a little more precision to your explanation: Angle of Attack is the angle between the chord line of the wing airfoil and the local relative wind
“Add precision”. What a polite way to straight up correct me . And thank you for that I had definitely misunderstood the definition. Thankfully the concept remains unchanged. Nice post.
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Old 04-08-2020, 06:38 AM   #6
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Well thanks for the explanations. I can say that I already knew most of it ;-) I would like to point out that although the AOA is measured based on the wings chord line, we are still talking about a FIXED wing aircraft which means that the angle between the wing chord line and the aircraft fuselage does not change.

I understand now that the AOA indexer shows arbitrary units, but I would still like to know whether it's possible to convert the unit shown by the AOA indexer to actual degrees or mils. Is this a logarithmic or linear scale?

The reason I ask is, that I would like to calculate/verify the calculation for the pipper depression for manual bombing and I have to take into account the angle between the flight path (aim off distance / aim off point) and the point where the nose of the aircraft is actually pointing at.
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Old 04-08-2020, 09:55 AM   #7
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I think, for ease of use gauge will show units of AOA that stall happen always at same value no matter the plane. So for each different planes actual AOA in degrees will differ.
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Old 04-08-2020, 11:20 AM   #8
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Units of AoA are all different for different aircraft. 15 units in the A-10 are different to 15 units in the F-15 for example....

It was all adjusted, so no matter what aircraft you step in, when you see 15 units of AoA, you have the best AoA for landing.... which would be different for different aircraft.

Why they chose this system .... my guess is as good as yours, but maybe in the 60-70-80s they thought that pilots would be changing airframes a lot more frequently
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Old 04-08-2020, 12:23 PM   #9
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Quote:
Originally Posted by Shadow KT View Post
It was all adjusted, so no matter what aircraft you step in, when you see 15 units of AoA, you have the best AoA for landing.... which would be different for different aircraft.
But is that so? And does a stall always occur around 30 units of AoA? I still find it hard to find good solid information on this.
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Old 04-08-2020, 03:42 PM   #10
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Wouldn't it be easier to just show degrees of AOA?
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